A continuous casting mold flux is a powdery or granular auxiliary material used in steel making for covering the molten steel surface in a crystallizer of a conticaster. Due to high temperature of the molten steel, the mold flux comprises a solid layer and a liquid layer, wherein the molten layer is immediately adjacent to the molten steel, and the part of the mold flux above the molten layer remains in its original granular or powder form so as to achieve good insulation and thus prevent the solidification of the molten steel surface. On the other hand, due to the periodic vibration of the crystallizer, the molten layer flows continuously into a crevice between a copper plate of the crystallizer and an initial shell of the molten steel to lubricate the relative movement between the shell and the copper plate, such that good surface quality of a cast slab is guaranteed. In addition, the molten layer can also absorb nonmetal inclusions floating in the molten steel and purify the molten steel. Generally, the mold flux film flowing into the crevice between the copper plate of the crystallizer and the shell is only 1-2 mm. One side of the film that is adjacent to the copper plate is in solid state, while the other side adjacent to the shell is still in liquid state. The liquid phase has a function of lubrication. The solid phase has good control over the capability of the copper plate of the crystallizer in cooling the shell, such that the cooling rate of the molten steel may be regulated and the controlled heat transfer can be achieved. Hence, a mold flux is the last process technique for controlling the surface quality of a cast slab in steel making. A mold flux with inappropriate properties may induce surface deficiencies such as flux inclusions, cracks, etc. to the cast slab. More seriously, the shell may even break and an accident of steel leakage may be incurred. Therefore, a mold flux is an important means for guaranteeing successful proceeding of a continuous casting process and surface quality of a cast slab.
A continuous casting mold flux is mainly a binary system of CaO and SiO2, accompanied with fusion aids such as CaF2, Na2O, Li2O and the like to lower melting point and viscosity of the binary system of CaO and SiO2, further with a small amount of such components as Al2O3, MgO, MnO, Fe2O3 and the like to obtain desirable metallurgical properties. Since the melting point of a mold flux is about 400° C. lower than the temperature of molten steel, an amount of carbonaceous material must be added to allow slow melting of the mold flux having a relatively low melting point on the molten steel surface. The carbonaceous material that has a very high melting point can stop agglomeration of liquid drops of the mold flux effectively, and thus retard melting of the mold flux. To the extent of these components of the mold flux, the ratio of CaO to SiO2 (i.e. CaO/SiO2, referred to as basicity hereafter) and the amount of F may be regulated to have an effective control over the crystallization rate of cuspidite (3CaO.2SiO2.CaF2) in order to fulfill the purpose of adjusting the mold flux reasonably and controlling heat transfer accordingly. Higher crystallization rate results in higher thermal resistance of the mold flux and lower heat transfer intensity. Fully vitrified mold flux has the minimum thermal resistance and the maximum heat transfer intensity. For low-carbon steel, ultralow-carbon steel and those types of steel having poor thermal conductivity (e.g. silicon steel, etc.), in order to reinforce cooling of cast slabs, crystallization of the mold flux is undesirable. Hence, the amount of F is generally low, specifically about 3-5%. However, for peritectic steel and those types of steel containing crack-sensitive elements, if the cooling of molten steel in a crystallizer is uneven or too fast, the initial shell will break readily at weak locations under various stresses, resulting in longitudinal cracks. For these types of steel, the mold flux must have a high crystallization rate to effect slow cooling and inhibition of cracking. In these circumstances, the content of F in the mold flux is often as high as 8-10%. It can be seen that F is used in a mold flux not only for lowering melting point and viscosity, but also plays an important role in increasing crystallization rate. Thus, F is an indispensable element in a traditional mold flux.
It is well known that F is a toxic element whose harm to human beings, animals and plants is at a level 20 times higher than the harm level of sulfur dioxide. Due to high working temperature of the mold flux, generally about 1500° C., a large quantity of environmentally harmful fluoride gases (including SiF4, HF, NaF, AlF3, etc.) are produced in melting process. Fluorides in air, especially HF, are among the common air pollutants. Additionally, after exiting the crystallizer, the molten mold flux that has high temperature contacts with secondary cooling water sprayed on a cast slab at high speed, and they interact with each other to undergo the following reaction:2F−+H2O=O2−+2HF
When HF dissolves in water, fluoride ion concentration and pH of the secondary cooling water are increased. As the secondary cooling water is recycled, fluoride ions will be further enriched, and pH will be further increased. The increase of the fluoride ion concentration and pH of the secondary cooling water accelerates corrosion of the continuous casting equipment greatly, leading to higher maintenance fee of the equipment, higher difficulty and neutralizer cost in treatment of the recycling water, and higher burden of sewage discharge.
In view of the above problems concerning a F containing flux, both domestic and foreign metallurgists have been devoting themselves actively to the development of environmentally friendly mold fluxes free of F. At present, a relatively feasible solution involves replacement of F with B2O3, Li2O, and a suitable combination of which with Na2O effects adjustment of the melting properties of a mold flux. Japanese patent publications JP2007167867A, JP2000169136A, JP2000158107A, JP2002096146A and Chinese patent application CN201110037710.8 disclose solutions in which no B2O3 or a small amount of B2O3 is added. According to these solutions, the melting point or the viscosity of the mold flux is generally rather high. Specifically, the melting point is higher than 1150° C., or the viscosity at 1300° C. is higher than 0.5 Pa·s. Unduly high melting point or viscosity renders consumption of liquid flux excessively low, which is unfavorable for cast slab quality and smooth proceeding of a continuous casting process. In order to develop a fluoride-free mold flux being valuable in industrial application, the cost of raw materials has to be taken into consideration. Inasmuch as Li2O is expensive, the technology using B2O3 in replace of F is most promising for application. Because the melting point of B2O3 is only on the order of 450° C., far lower than those of the other components of a boron-containing mold flux, the softening temperature of the solid phase of the mold flux is apparently rather low. Consequently, the proportion of the solid phase in the flux film located in the crevice between the copper plate of the crystallizer and the shell is rather low, resulting in lowered thermal resistance of the flux film and rather high heat flow in the crystallizer. In addition, B2O3 in the mold flux tends to have a network structure, which inhibits crystallization. As a result, the solid phase has a vitreous structure. A vitreous solid phase has lower thermal resistance than a crystalline solid phase. Therefore, a boron-containing flux has lower thermal resistance than a traditional fluoride-containing flux. Once the excessively high heat flow exceeds the limit designed for a caster, not only the service life of the crystallizer will be affected, but the risk of sticking breakout will be increased. Hence, the heat flow must be curbed. Under normal conditions, a crystallizer in a continuous slab casting process has a comprehensive heat transfer coefficient of 900-1400 W/m2K. Additionally, the comprehensive heat transfer coefficient increases as the draw speed is increased. Thus, in the case where a boron-containing flux is used in production, the comprehensive heat transfer coefficient of the crystallizer will reach an upper limit of 1300-1400 W/m2K when the draw speed is 1.0 m/min. However, the draw speed of existing domestic and foreign slab casters in operation is basically 1.2 m/min. For low-carbon steel and ultralow-carbon steel, the draw speed is even up to 1.6 m/min or higher. When these types of steel are concerned, a normal production rhythm can hardly be realized using a boron-containing, fluoride-free flux. This deficiency has to be remedied by enhancing the crystallization rate of the boron-containing flux. Japanese patent publication JP2001205402A and Chinese patent application CN200510065382 disclose boron-containing, fluoride-free fluxes, but crystallization rate is not taken into account. Hence, the mold fluxes must face the risk of unduly high heat transfer property during use. The mold flux disclosed by Chinese patent application CN200810233072.5 has an excessively high crystallization rate, and thus it is only adapted to crack-sensitive steel such as peritectic steel, etc. Chinese patent application CN03117824.3 proposes perovskite (CaO.TiO2) as the subject of crystallization. However, the melting point of perovskite is higher than 1700° C., which is unfavorable for lubrication. Thus, its prospect of application is limited. The mold flux designed in Chinese patent application CN201010110275.2 uses a composite crystalline phase of merwinite and sodium xonotlite. However, its viscosity is rather high, and thus it is more suitable for a billet continuous casting process.
As mentioned above, F, as an indispensable component in a traditional mold flux, has the function of lowering melting point and viscosity of the flux, and is an important means for controlling heat transfer in a continuous casting crystallizer. However, due to its harm to human health, pollution of atmosphere and water, and accelerated corrosion of equipments, it is a research subject on which those skilled in the art are concentrated to obtain a fluoride-free continuous casting mold flux. The cost of a mold flux free of fluoride is also an important concern that must be considered for its industrial application on a large scale. Currently, substitution of B2O3 for F is the most economical and feasible technical concept. The biggest deficiencies of a boron-containing flux include its low crystallization rate and lowered softening point of solid phase, resulting in small thermal resistance of the boron-containing flux in use and excessive heat transfer of a continuous casting crystallizer, which is unfavorable for increase of the draw speed of a conticaster and restricts the output of a steel plant. The inventors of the present invention have developed a boron-containing, fluoride-free flux having a moderate crystallization rate, which can be used in a crystallizer to control transfer of heat from molten steel effectively, and has been applied successfully in a low-carbon steel slab conticaster.